As a collective antenna device that has a plurality of antenna elements housed in one case, one that is described in Patent Literature 1 is known. The device disclosed in Patent Literature 1 encases an antenna used for automotive wireless communication, and antennas for receiving high-frequency signals used in satellite digital audio radio service (SDARS) inside a housing. There are two antennas for SDARS, one being the antenna for receiving radio waves emitted from a satellite, and the other being an antenna for receiving terrestrial radio waves transmitted from a ground relay station.
The automotive wireless communication envisioned here uses either 900 MHz or 1.8 GHz frequency band, while SDARS uses 2.3 GHz frequency band. Therefore, the antenna for the automotive wireless communication is longer than the antennas for SDARS.
The device disclosed in Patent Literature 1 is attached to a rear part of the roof of a car. The case is formed such that its internal spatial height continuously increases from the front end to a point close to the rear end so as to reduce air resistance. Due to this shape, the internal spatial height of the case is relatively greater in the rear part.
In Patent Literature 1, this internal spatial height in the rear part is used to accommodate the antenna for the automotive wireless communication that is relatively longer due to the relatively lower frequency band.
However, most cars have a roof that is highest in the middle in the front-back direction and inclined from the peak toward the rear end of the roof. Since the collective antenna device is commonly installed in the rear part of the car roof, the radio waves emitted from the collective antenna device at low elevation angles in the forward direction of the vehicle are blocked by the inclination of the roof from near its center toward the rear end. The higher the frequency, the more straight, the radio waves travel. Thus, the higher the frequency of emitted radio waves, the more the radiation at low elevation angles in the forward direction of the vehicle is reduced.
Therefore, when the roof has such a shape, the technical issue encountered by the device of Patent Literature 1 was that the power gain of each antenna in the collective antenna device was reduced, in particular, the power gain in the forward direction of the antenna for relatively high-frequency terrestrial radio waves of SDARS would drop largely, because of which the radiation level of the antenna was low.
The technical issue of reduced power gain in the forward direction of an antenna that emits radio waves at relatively high frequencies arises not necessarily with antennas used in automotive wireless communication or SDARS but when there is an inclined configuration that blocks radiation in front of a collective antenna device that includes a plurality of antennas emitting radio waves of mutually different frequencies such as a mobile network antenna and an antenna used for vehicle-to-vehicle communication or road-to-vehicle communication.